1. Field of the Invention
The invention relates to the field of electrochromic elements which are often fitted onto or between transparent substrates and which facilitate the variation in transparency of such systems when an electric current is applied.
2. Background of the Invention
Electrochromic elements known from prior art most often consist of two transparent substrates, for example, made from glass or plastic. The substrates are connected to each other by way of a centrally placed ion conducting component, which is surrounded on both sides by an electrode and an electrochromic coating. Appropriately, the electrodes are also made from a transparent material, which is possible when using, for example, doped metal oxides such as aluminium-doped zinc oxide, fluoride-doped tin oxide or also indium-doped tin oxide-TCO materials in general. Materials used for a first electrochromic coating (EC1) are, for example, tungsten oxide, molybdenum oxide, nickel oxide or also iridium oxide. Materials used for the second electrochromic coating (EC2) could, for example, include cerium oxide, titanium oxide, vanadium oxide, niobium oxide, tantalum oxide or also mixtures of the above mentioned metal oxides. The ion conducting component is often manufactured on the basis of modified polyvinyl butyrals, polyethylene oxides, polyethyleneimines or polyacrylates.
The use of modified polyvinyl butyrals is known from DE 103 27 517 and WO 02/40578. Patent documents FR 2,690,536, U.S. Pat. No. 5,241,411 and DE 692 138 33 relates to the use of modified polyethylene oxide and polyethyleneimine for the same purpose. The use of acrylates and other polymers is already well known from EP 1,283,436, DE 695 281 48 and DE 4 417 219.
Electrochromic elements having an ion conducting component accordingly typically have the following structure:
Substrate-TCO-EC1-Ion Conducting Component-EC2-TCO-Substrate
Electrochromic elements of this construction will change transmissivity when an electric current is applied to them. If the electrochromic element components EC1 are negatively charged by the relevant TCO electrode the electrochromic element will darken. If the polarity is then reversed (i.e., if a positive charge is applied to the EC1 component and a negative charge is applied to the EC2 component), then decolorization or bleaching (i.e., an increase in transmissivity in comparison to the original setting of the electrochromic elements) will occur.
An important characteristic of this electrochromic element, which is already well known in prior art, is that an electric current must be applied to the electrochromic components to change optical transmissivity—both to decrease and increase transmissivity, but not to maintain it.
Other implementations of electrochromic elements are also known but they have the disadvantage that a permanent current must be applied not only to increase transmissivity, but also to maintain it. On the other hand, such electrochromic elements consist of two substrates which each are fitted with a transparent electrode on one side, for example, in the form of a TCO material and a gel containing redox pairs which locks the substrates in between it in such a way that both sides of the gel coating are in contact with a TCO electrode. The gel coating containing redox pairs has viologenes, which can reduce or oxidize when an electric current is applied. This chemical conversion process occurs in connection with the electrochromic element changing its transmissivity. This implementation of an electrochromic element also has the disadvantage that each reverse reaction of the previously induced oxidization or, as the case may be, reduction spontaneously ends when the electric current is removed and therefore always results in an increase of transmissivity. This is advantageous if such electrochromic systems are used in, for example, cars. In this case it is important that these electrochromic elements automatically will bleach or switch to a highly transmissive state when the current supply is cut off. Such electrochromic elements on the basis of viologenes and other redox pairs are known from U.S. Pat. No. 5,998,617 and DE 60 003 773.
A gel with electrochromic elements having redox pairs typically has the following structure:
Substrate-TCO-Gel with Redox Pairs-Substrate
In comparison to electrochromic elements with an ion conducting component that employ an electric current to adjust transmissivity which does not change at all or only little, a gel with electrochromic elements having redox pairs bleaches or increases in transmissivity a few seconds or minutes after the electric current has been removed. Electrochromic elements having an ion conducting component are thus much more stable and resistant to self-bleaching than a gel with electrochromic elements having redox pairs. However, after the current has been removed the stability in the low transmissivity state for the first-mentioned electrochromic element is only guaranteed at moderate temperatures under 30-35° C. In comparison, in the implementation of electrochromic elements at higher temperatures of, for example, 50° C., a constant increase in its transmissivity occurs even without applying an electric current. Furthermore, the speed of the bleaching for these electrochromic elements is characterized by being distinctly temperature dependent; the higher the temperature the electrochromic element is exposed to, the faster transmissivity rises. Particularly in the summer months, the operating temperatures of architectural glass having electrochromic elements are between 50 and 60° C., often even between 70-80° C. At such high temperatures a corresponding rise in transmissivity is clearly evident. An electrochromic element having an ion conducting component also shows obvious bleaching within a few days. In practice this means that an operator has to darken the architectural glass having one electrochromic element at regular intervals to properly use the darkening effect. To do this effectively, it is necessary to make regular transmissivity measurements, which is impractical.
From a phenomenological standpoint, the rise in transmissivity occurs as a consequence of dispersement of freely mobile charge carriers. The dispersed charge is therefore no longer available for use by the operator of electrochromic elements to subsequently induce a reduction of transmissivity by means of an applied electric current.
It is the object of the invention to develop an electrochromic element that after its transmissivity has been lowered by being exposed to an electric current for a short time, it will stabilize to the adjusted transmissivity degree for a long period of time after the electric current has been removed even while being exposed to high temperatures, in other words, it will only show limited bleaching.